The Russian fleet cannot leave the Black Sea because Turkey will not let them transit the Bosporous. See the Montreaux Convention.
How much do you know about that League of Nations era treaty? Moscow has objected to it strongly since 1940s. Would Turkey risk a firefight over it? USA is not a signatory and IInited Nations has not successfully engaged. It is an old piece of paper. There are many things not being honored by world players, USSR was a signatory but it no longer exists as a country. Maybe Russia would feel bound, maybe not. That is a real risk and danger created by this Biden proxy war, among many.